Caffeine Reduces Deep-Sleep Brain Waves, EEG Study Finds — SkimNews
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- Wroclaw Medical University researchers found that caffeine can reduce slow-wave activity in the brain during sleep — even when people fall asleep normally and remain in bed for what appears to be a full eight hours — without the sleeper necessarily noticing.
- Prof. Donata Kurpas said quantitative EEG analysis reveals subtler changes than duration tracking, including reduced slow-wave activity, an important marker of sleep depth and its restorative character.
- Caffeine's effect on the sleeping brain shifts the EEG pattern toward a more 'wakeful' brain even when no awakenings occur, so a person's subjective sense of having slept well may not match what neurophysiological recordings show.
- Individual sensitivity to caffeine varies dramatically based on genetics, metabolism, age, stress, and chronic fatigue, meaning even coffee consumed earlier in the day may still affect nighttime recovery for some people.
- Caffeine can create a vicious cycle of greater daytime fatigue, greater need for stimulation, and poorer nighttime recovery — functioning like 'borrowing energy' from the body, according to Kurpas.
- The study was published as 'The Caffeinated Brain Part 2' by James Chmiel and Donata Kurpas in the journal Nutrients in 2026.
Why it matters: By reframing caffeine's harm from sleep onset to sleep quality, the review exposes a hidden deficit — people who feel rested may still be losing slow-wave recovery, and because genetics, age, and stress alter the response, blanket timing advice misses athletes and heavy caffeine users at risk of a fatigue-stimulation cycle.
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